If your string trimmer won’t start, spin, or feed line properly, the cause is usually a worn or clogged part that’s easy to replace once you know where to look. Understanding how a trimmer works, and the common failure points that stop it from doing its job, can save you time, frustration, and money. This article explains why your string trimmer might not start (from spark plug to carburetor issues), why the head might not spin (including clutch and shaft problems), and what to do if the trimmer line won’t feed. By the end, you’ll know exactly how to diagnose and repair your trimmer using the right replacement parts for your model.
The trimmer engine won’t start
While some string trimmers are electrically-powered using a power cord, the majority are cordless, using a battery or a gasoline engine to power the product. If your trimmer’s gas-powered engine won’t start, here are the most likely causes:
- Defective spark plug – Due to carbon build-up or a weakened electrode, a spark plug will degrade over time and be unable to ignite the compressed fuel and air inside the engine’s combustion chamber to start or run the engine. You can use an ignition tester to help determine the strength of the spark plug, but it’s probably easier to simply replace it with a new one, something we recommend doing as part of an annual string trimmer engine tune-up.
- Restricted carburetor – Old or bad fuel can leave a gummy residue inside the carburetor creating a restriction. This will prevent the proper ratio of fuel and air from entering the cylinder which can cause the engine to have trouble starting. You should always make sure you keep fresh fuel in the tank and use a fuel stabilizer to help preserve its quality. You can try cleaning the carburetor ports with a dedicated carburetor cleaner, or WD-40, to clear out the restriction, but if the clog is particularly bad, you may need to purchase a new carburetor.
- Clogged fuel filter – That gummy residue that can clog a carburetor can do the same to the fuel filter. Simply replace the filter with a new one to solve the problem. Again, this is something that is recommended anyway as part of the engine’s annual maintenance.
- Malfunctioning recoil starter – The recoil starter may be unable to engage with the trimmer engine crankshaft, or the rewind spring inside the starter assembly could be broken. The spring can often be replaced on its own, but it might be easier to replace the entire recoil starter assembly even if the components apart from the spring are in good working condition.
- Clogged spark arrestor – The spark arrestor is a thin metal mesh designed to prevent sparks emitted by the engine from exiting the muffler and potentially starting a fire. Over time the arrestor can become clogged with debris and affect engine performance. You can try cleaning the arrestor with a wire brush or replace the part with a new one.
- Dirty air filter – By restricting the air coming into the carburetor, a dirty air filter can prevent the engine from starting as well. Yet another part that should always be replaced annually when tuning-up the engine.
The trimmer head won’t spin
If you have no problem starting the engine, but the trimmer head won’t spin when you depress the throttle trigger, these could be the reasons:
- Worn clutch – The clutch engages with one or more drive shafts attached to the trimmer head to rotate the head when the trigger is pulled. If the clutch is worn out, the trimmer head may not spin. A new clutch assembly is relatively inexpensive and easy to install.
- Damaged drive shaft – A similar situation to a worn-out clutch occurs if a drive shaft is damaged. Again, a new shaft can usually be purchased individually to repair the string trimmer.
- Defective trimmer head – The trimmer head itself could be defective and unable to spin. This is another replacement part that can be purchased separately. Keep in mind, if the threads of the drive shaft are stripped, you’ll want to replace both the head and shaft at the same time.
- Broken throttle cable – If the throttle cable is broken, the engine won’t accelerate when the trigger is pulled and the clutch won’t engage the drive shaft to turn the head. As with the items listed above, a new throttle cable can be purchased separately to repair the trimmer.
The trimmer line won’t feed
So the trimmer engine is running smoothly and the trimmer head is spinning… and, yet, no new line is coming out of the head. Here are the top reasons why:
- Worn-out trimmer head – Just because the head can spin doesn’t mean it’s in good enough condition to properly feed the trimmer line. Before replacing, first try to clear out any grass or debris that may be clogged in the assembly; you should fully remove the head to clean it thoroughly.
- Damaged trimmer head housing – On many models, the trimmer head is secured in a separate housing. If the housing is cracked or chipped, the line may be unable to feed.
- Defective bump knob – Many trimmers will use a bump knob that can be tapped on the ground to activate a spool inside the trimmer head to issue more line. If the bump knob is defective (or the spool is damaged, for that matter), the line won’t feed properly. Again, both of these parts can be replaced to fix the problem.
- Wrong size trimmer line – Not all trimmer line is equal as the size and thickness of the line varies depending on the trimmer model. The most common diameters for residential use range from 65/100th of an inch to 95/100th of an inch, but you need to make sure you’re using the right line for your model. Otherwise, the line can become jammed inside the head.
Find the right string trimmer parts with Repair Clinic
Repair Clinic stocks all the parts you need to tune-up or repair your string trimmer’s engine, as well as the parts to keep the trimmer head spinning and the line feeding. To get started, enter the full model number of your string trimmer in the Repair Clinic website search bar. You can then use the part category and part title filters to refine the search to identify the specific part you need, whether it’s a carburetor, rewind spring, clutch, throttle cable, trimmer head, bump knob, or trimmer line. Repair Clinic carries string trimmer parts for all the top brands, including Craftsman, Echo, Husqvarna, Kawasaki, MTD, Poulan Pro, Ryobi, Toro, and Troy-bilt, but you’ll want to make sure you’re purchasing the exact part that matches your string trimmer model.